Magnesium alloy



Patented Nov. 7, 1939 UNITED STATES PATENT OFFICE MAGNESIUM ALLOY NoDrawing.

1936, Serial No. 112,288.

Original application November 23,

Divided and this application August 7, 1939, Serial No. 288,768

3 Claims. (01. 75-168) This invention relates to magnesium alloys andparticularly to those containing magnesium in excess of approximately 80per cent. 7

It is well known that the addition of certain metals to magnesiumresults in the production of alloys possessing good strengthcharacteristics. while the addition of other metals results in theproduction of alloys possessing good corrosion resistance. One of themajor problems of the magnesium industry is the preparation of an alloyin which both objectives are obtained at the same time.

Accordingly, the object of the present invention is to prepare magnesiumalloys which have improved physical properties and corrosion resistance.Other objects and advantages will appear as the descrlption'proceeds.

My invention is based on the discovery that highly desirable and usefulalloys can be prepared by the addition of silver to magnesium andmagnesium alloys, and that these alloys possess ood strengthcharacteristics, combined with good corrosion resistance.

1 have discovered that the strength characteristics of magnesium may beimproved to a very marked degree by the addition of suitable amounts ofsilver. The tensile strength of cast pure magnesium, for example, wasfound to be 14,000 pounds per square inch, while that of a magnesiumalloy containing 2 per cent of silver was 21,100 pounds per square inchand that of a magnesium alloy containing 4 per cent of silver was 22,300pounds per square inch. "the yield strength of cast magnesium was 2,500pounds per square inch, while the yield strengths of the magnesiumalloys containing 2 per cent of silver and 4 per cent of silver were4,100 pounds per square inch and 5,100 pounds per square inchrespectively. The addition of 2 per cent and 4 per cent or" silver topure magnesium raised the percentage elongation of the cast metal from8.0 per cent up to 10.5 per cent and 10.0 per cent respectively. Thetoughness of the magnesium was likewise improved by the addition ofsilver. The single blow impact value for pure magnesium was 9.0

' P oportional decrease in the percentage elonga tion and toughness. Themagnesium alloy conalloy containing 12 per cent of silver had a Brinellhardness value of 51.

Magnesium-silver alloys, particularly those containing approximately 4per cent or more of silver, are amenable to heattreatment. A solutionheat treatment of 18 hours at 770 F. in' creased the tensile strength ofthe magnesium alloy containing 4 per cent of silver from 22,300 poundsper square inch to 24,900 pounds per square inch, and increased thepercentageelongation from 10.0 per centto 11.5 percent withoutappreciably affecting other properties- A subse quent precipitation heattreatment of 48 hours at 350 F. produced a small increase in the tensilestrength. Precipitation heat treatment, however, produced markedproperty improvements, particularly in tensile strength, yield strength,and hardness in alloys containing higher percentages of silver. Asolution heat treatment of 18 hours at 770 F. followed by aprecipitation heat treatment of 48 hours at 350 F., for example,increased the Brinell hardness of the magnesium alloy containing 8 percent of silver mum properties in the heat treated condition arerequired, and then I prefer to use alloys containing approximately 5 percent to 8 per cent of silver. Alloys with low percentages oi silver arebetter adapted for plastic deformation operations, while alloys withhigher percentages of silver are better suited for the production atcastings.

Furthermorai have discovered that the acidi tion of silver to commercialmagnesium is henc ficial from the standpoint of corrosion resistance.This may be illustrated by alternate ersion corrosion tests conducted ina 3 per cent salt so lution. At the end of 24 hours, pure magnesium hadlost weight at the rate or 65.8 mg/cm lday, while the magnesium alloyscontaining 2 per cent of silver and 4 per cent of silver lost only 51.2and 17s mg/cm /day respectively. in another test the addition or 1 percent of silver produced a per cent decrease in the corrosion rate ofmagnesium.

have also discovered that the beneficial .words, when silver is added toan alloy consisting of magnesium with at least one of the metalsaluminum, manganese, zinc. In such alloys, the percentage of silver mayvary from about 0.3 per cent to. 12 per cent, the percentage of aluminummay vary from about 0.5 per cent to 12 per cent, the percentage of zincmay vary from about 0.5 per cent to 8 per cent, and the percentage ofmanganese may vary from about 0.1 per cent'to 2 per cent (0.1 per centto 0.5 per cent in alloyslikewise containing aluminum), but the totalpercentage of added metals should not exceed approximately 20 per cent.The absolute percentage of each metal is dependent upon the use forwhich the alloy is intended and upon the percentages of the otheralloying ingredients. For plastic deformation processes, I normallyprefer from 0.5 per cent to 4 per cent of silver, from 0.5

per cent to 6 per cent of aluminum, from 0.2 per cent to 0.8 per cent ofmanganese, and from 0.5 per cent to 2.0 per cent of zinc, with a maximumof approximately 8 per cent 01F added metals. For castings, I normallyprefer from 0.5 per cent to 8 per cent of silver, from per cent to percent of aluminum, from 0.1 per cent to 0.4 per cent of manganese, andfrom 1 percent to 4 per cent of zinc, the percentage of total addedingredients varying from approximately 8 per cent-to 12 per cent.

The following examples serve to illustrate the beneficial effect ofsilver in this class of magnesium alloys as expressed by improvements inphysical-mechanical properties. For example, the ternary magnesium alloycontaining 2 per cent of silver and 0.2 per cent of manganese had atensile strength of 23,000 pounds per square inch as compared with18,000 pounds per square inch for the binary magnesium alloy containing0.2 per cent, of manganese and 21,100 pounds per square inchfor thebinary magnesium alloy containing 2 per cent of silver. The yieldstrength of this ternary magnesium-silver-manganese alloy was 4,400pounds per square inch'as compared with 3,000 pounds per square inch forthe binary magnesium alloy containing 0.2 per cent of manganese and4,100 pounds per square inch for the binary magnesium alloy containing 2per cent of silver.

The addition of silver to magnesium-aluminum and tomagnesium-aluminum-manganese alloys has been found to be particularlybeneficial when the alloys are used for the production of heat treatedcastings. Under such conditions, comparable physical propertyimprovements are obtained in alloys with and without manganese, butotherwise of similar composition, although the presence of manganese isdesirable when the alloy is used for extrusion purposes. The addition of2 per cent of silver, for example, increased the tensile strength of asolution heat treated magnesium alloy containing 8 per cent of alumi numand 0.2 per cent of manganese from 34,000 pounds per square inch to361000 pounds per square inch and the yield strength of this same alloyfrom 11,000 pounds per square inch to 12,000 pounds per square inch.Maximum improvement due to the presence of silver in alloys containingaluminum and manganese occurs in the aged or precipitation heat treatedalloys, and here the effect is particularly pronounced in the yieldstrength and hardness values. For example, the addition of 2 per cent ofsilver 130 thi silver-manganese-zinc alloys are satisfactory for theproduction of wroughtshapes. While both alloys can be extruded readily,the former is better for rolling and forging operations, since theannealing temperature generally employed in such operations closelycorresponds to the temperature at which precipitation of manganeseoccurs, and this precipitated manganese lowers the ductility andworkability of the alloy. Magnesium-silver-zinc alloys, suitable forrolling, contain approximately 0.5 per cent to 3 per cent of silver and0.5 per cent to 1 per cent of zinc, the balance being magnesium.Magnesiumsilver-zinc alloys, suitable for extrusion, containaproximately 0.5 per cent to 6 per cent of silver and 0.5 per cent to 5per cent of zinc, the balance being magnesium. Where ease of subsequentdeformation is not required, as, for example, in extruded sections, Inormally prefer to use magnesium-silver-manganese-zinc alloys containing0.5 per cent to 5 per cent of silver, 0.1 per cent to 0.6 per cent ofmanganese, and 0.5 per cent to 6 per cent of zinc. The properties ofsuch alloys, particularly those containing more than approximately 3 percent of silver and 2 per cent of zinc, may be further improved by heattreatment.

Although the magnesium-silver-zinc alloys may be used for the productionof castings, I generally prefer to use, for such purposes, alloyscontaining silver, aluminum, and zinc, or silver, aluminum, zinc, andmanganese.

Manganese is soluble in this type of alloy to the extent of a, fewtenths of a per cent, and in such amounts has no appreciable-effect onphysi cal property improvement, although it does improve the corrosionresistance of the alloy. The choice between the alloy with or withoutmanganese, but otherwise of similar composition, depends largely on theuse for which the product is intended. A good composition, selected fromthe more corrosion resistant type of alloy, contains 2 per cent ofsilver, 8 per cent of aluminum. 0.2 per cent of manganese, and 3 percent of zinc. This alloy, in the solution heat treated condition, has atensile strength of 27,200 pounds per square inch, a yield strength of13,000 pounds per square inch, 4 per cent elongation, a Brinell hardnessvalue of 53, and a single-blow impact value of 9.7 foot pounds. In thesolution-precipitation heat treated condition, this same alloycomposition has the following properties,-tensile strength 31,000 poundsper square inch, yield strength 2l,000 pounds per square inch, 0.5 percent elongation, 72 Brinell hardness, and a singleblow impact value of2.5 foot pounds.

I have likewise discovered that the corrosion V resistance of themagnesium-silver alloys may be improved very materially by the additionof at least one of the metals aluminum; manganese, zinc. For example,the loss in weight in a magnesium alloy containing 2 per cent of silverwhen tested in 3 per cent salt solution was 51.2 mg/cm /day, whereas theaddition of 0.2 per cent of manganese reduced this corrosion rate to 4.9mg/cm /day. In another instance, the addition of 0.9 per cent ofmanganese to a binary magnesium alloy containing 1 per cent of silverreduced the corrosion rate from approximately 60 mg/cm /day to 1.7'mg/cm /day. In a similar fashion, the addition of 8 per cent ofaluminum plus 0.2 per cent of manganese to a binary magv nesium alloycontaining 2 per cent of silver reduced the corrosion rate from 51.2mg/cm /day to 1.2 mg/cm /day, while the addition of 8 per cent ofaluminum plus 0.2 per cent of manganese plus 3 per cent of zinc to thebinary alloy containing 2 per cent of silver reduced the corrosion rateto 0.6 mg/cm /day.

The above described alloys may be prepared by the well known methods ofalloying metals with magnesium, such as adding the respective alloyingingredients to a bath of molten metal protected from oxidation by acover of a fluid flux. The various alloying ingredients may'be addedsingly or simultaneously, and are usually added as pure metals, exceptin those compositions containing both aluminum and manganese, in whichcase these two metals are preferably added in the form of a 90-10aluminum-manganese hardener.

This is a division of my co-pending application Serial No. 112,288 filedNovember 23, 1936.

Other modes oi. applying the principle of my invention may be employedinstead of those explained, change being made as regards the ingredientsand the steps herein disclosed, provided those stated by any of thefollowing claims or their equivalent be employed.

I particularly point out and distinctly claim as my invention:

1. A magnesium base alloy comprising from 0.3 to 12 per cent of silverand from 0.1 to 2 per cent of manganese, the balance being'magnesium.

2. A magnesium base alloy comprising from 0.5

to 8 per cent of silver and from 0.2120 0.8 per cent of manganese, thebalance being magnesium.

3. A magnesium base alloy comprising about -8 per cent of silver andabout 1 per cent of manganese, the balance being magnesium.

JOHN A. GANN.

